Not sure if you've already seen this, but many articles in there worth reading, and give advice specifically about this. http://wiki.polycount.com/wiki/Freelance
You looking at them as a potential freelance client? My advice would be same as Ben's here: https://polycount.com/discussion/comment/1772256/#Comment_1772256 See also: http://wiki.polycount.com/wiki/Freelance#Invoicing
I'm sure someone in the industry will give a better insight, but in the meantime my assumption would be that you'd work at a similar pace to the time estimates in the freelancing section of the wiki. http://wiki.polycount.com/wiki/Freelance I don't think it mentions environments though.
1. http://wiki.polycount.com/wiki/Normal_Map_Modeling#Low-Poly_Mesh 2. http://wiki.polycount.com/wiki/Texture_atlas 3. Several ways to do ambient occlusion for games: a. Create a 2nd UV set for static baked lighting. http://wiki.polycount.com/wiki/Light_map, http://wiki.polycount.com/wiki/Ambient_occlusion_map. b. Bake AO…
Some links to help you understand the process. http://wiki.polycount.com/wiki/EnvironmentSculpting http://wiki.polycount.com/wiki/Modular_environments http://wiki.polycount.com/wiki/ModelingRubbleIntoAPath